A motor starter has three jobs split across separate devices: a breaker or fuse for short-circuit protection, a contactor to switch the motor on and off, and an overload relay to protect the windings from sustained overload. All three are sized from the motor's full-load current. The breaker is set high so the starting in-rush doesn't trip it; the overload is set near full-load current; and the contactor must be rated for motor-switching (AC-3) duty.
Reviewed: June 19, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: NEC (NFPA 70) Article 430, IEC 60947.
Safety notice. Motor protection is governed by detailed code rules (NEC 430, IEC 60947) that depend on motor type, service factor, duty and starter. These are first-pass guide values; final selection must use the device manufacturer's data and be verified by a licensed engineer. See our disclaimer.
How a starter is sized
The big gap between the overload (≈ FLA) and the breaker (≈ 2.5× FLA) is deliberate: the breaker must ignore the 6–8× starting in-rush, while the overload protects the windings during running. They guard against different faults.
Worked example — 11 kW DOL starter
Scenario: 11 kW, 415 V, 3-phase motor, PF 0.85, efficiency 90%, direct-on-line.
Overload set to about 20 A. Contactor: next AC-3 rating ≥ 20 A = 25 A. MCCB: 2.5 × 20 = 50 A → 50 A inverse-time breaker, which lets the ≈130 A in-rush pass at start but still protects against a short circuit. Get the FLA itself from the motor parameters calculator.
Frequently Asked Questions
From the motor FLA: overload ≈ 100–115% FLA, contactor AC-3 ≥ FLA, breaker ≈ 2.5× FLA (NEC 430.52). For a 20 A motor: overload ≈ 20 A, contactor 25 A AC-3, breaker ≈ 50 A.
Use the AC-3 (motor duty) rating, at or above FLA. AC-3 ratings are lower than AC-1, so read the AC-3 column. In star-delta, each contactor carries about 58% of line current.
Set the thermal overload to the nameplate FLA, adjusted for service factor — about 100% for SF 1.0 and up to 115–125% for higher SF (NEC 430.32). It protects against running overload, not short circuit.
The breaker only does short-circuit/ground-fault protection, so it is set high (≈250% FLA) to let the 6–8× starting in-rush pass. The overload, set near FLA, handles running overload. Different faults, different devices.
DOL connects full voltage — simplest, highest in-rush (6–8× FLA). Star-delta starts in star (≈⅓ current and torque) then switches to delta, cutting the surge. Soft starters and VFDs reduce in-rush further and electronically.